During the process of drilling an oil or gas exploratory or development well borehole, it is necessary to determine where the drill bit is located at all times and the bit's deviation from the prescribed path. Depending on the type of well, such as vertical or directional bore, it is imperative that down hole measurements such as direction and deviation angle relative to a vertical axis be measured accurately and frequently.
Mechanical autonomous devices presently used to measure deviation angle “only”, include pendulum pin prick mechanisms coupled to a mechanical timer housed in a relatively slender tube commonly referred to as one shot deviation recorders. The mechanical timer is set at the surface of the well bore prior to dropping the tube down the central bore of the drill string. The mechanical timer is manually set to a predetermined time for activation of the pendulum pin prick mechanism at approximately the same time the slender tube containing the timer and the pendulum pin prick mechanism reaches the bottom of the borehole. The pendulum pin prick mechanism, when activated by the mechanical timer, causes a pin prick hole to be formed in a paper target at an angle congruent with the angle the pendulum pin prick mechanism is deviated from vertical. The paper target has concentric rings representing degrees of deviation, printed on the surface exposed to the pendulum pinprick mechanism. The slender tube containing a pendulum pinprick mechanism and a mechanical timer is either retrieved by wire line or “tripped” out of the borehole with the drill pipe. The paper target is then retrieved from the pendulum housing and inspected. The deviation angle in degrees is then estimated by determining the location of the pinprick hole formed by the pendulum pinprick mechanism and the nearest printed concentric circle on the paper target.
The pendulum pin prick mechanism and a mechanical timer method of retrieving borehole measurements is an industry standard that is the most commonly used method of inexpensive and “quick check” of borehole measurements. However, the use of this method requires that all drilling activities cease and creates a downtime situation that leaves the drilling operators exposed to problems such as “stuck pipe”, lost circulation, or “blowout’, occurring in open hole conditions. The potential always exists of a premature timer activation or non-activation, and, since this is a “one shot only” method, such failure would require the method be repeated resulting in additional downtime costs. Other problems exist with the pendulum pinprick mechanism and its mechanical timer method including limited angle range of the instrument and the fact that the resulting paper target is simply an estimate and thus open to interpretation.
Other methods employed by the oil and gas industry to measure borehole parameters is the gyroscopic deviation angle or “Gyro Multi-shot Deviation Angle”. The Gyro Multi-shot instrument consist of a magnetic compass and tilt indicator mounted above a spinning gyroscope. A camera with a timed shutter release is mounted so that multiple pictures can be taken of the magnetic compass reading and the tilt indicator. The operator at the surface then attaches the Gyro Multi-shot instrument to a “wire line” and lowers it into the borehole. The Gyro Multi-shot instrument is stopped at the desired depth where the timed shutter release is activated and a picture of the compass reading and tilt indicator is taken. The process is repeated until desired deviation angles are completed and the Gyro Multi-shot instrument is retrieved. The camera film is then retrieved, developed and analyzed. Although the Gyro Multi-shot instrument is accurate and reliable, development of the camera film and analysis of the deviation angles can take considerable additional time and is also vulnerable to the same problems of the “pendulum pin prick” mechanism method. In addition, the multi-shot method requires trained operators, thereby incurring additional cost.